A 1-month old “beautiful” baby girl was killed Thursday night in an apartment fire likely set to cover up the murder of her mother, who was found dead with multiple stab wounds, police sources said.

Horrified firefighters — who believed they were responding to a routine apartment blaze — tried desperately to revive the infant, performing CPR on her tiny body on top of a car on the street, authorities said.

But she was pronounced dead of smoke inhalation at Jacobi Hospital shortly after.

The 30 year-old mother, whose name was withheld, was dead on the scene.

The woman was stabbed repeatedly in the torso and arms, then dragged to a bathroom and tossed under a mattress, which was set ablaze, according to the FDNY and sources.

“The bathroom was lit on fire, possibly to cover up the crime scene,” said Joe Saccente, FDNY Deputy Fire Chief.

The infant was found in the bathroom in a tub, according to the FDNY.

A police source said that they are treating the heinous crime as a double murder.

Cops are looking to question the woman’s boyfriend, the source said.

It’s unclear if that boyfriend is the baby’s father.

“She looked like she’d been through a lot,” said one neighbor.

The dead woman had been a victim of domestic violence in the past and had another child who was under the care of the city’s Administration for Children’s Services, said the neighbor.

Neighbors said she was devoted to the newborn.

“I saw her like a week ago, and I was telling her how beautiful she was,” said one.

“She was very tiny.”

The gruesome crime stunned residents in the five-story Kingsbridge building.

“It’s devastating that this baby could perish,” said Kizzy Marte, who lives across from the charred apartment.